It is known that potassium chloride crystals can be recovered from feed solutions which contain sodium chloride and other impurities by cooling the solution under a vacuum. In this operation, water is evaporated and the latent heat of evaporation is removed from the solution to be cooled. The steam which is generated is used to heat the process liquor (German patent publication No. 10 59 418).
A disadvantage of these and similar processes is that the crystals resulting from such cooling are smaller than those obtained from crystallization in upright vessels. Because additional quantities of water are evaporated, larger quantities of fresh process liquor must be produced. Besides, the potassium chloride which was recovered tends to harden in storage. Another disadvantage is that the crystals contain minor amounts of sodium chloride and process liquor adheres to the crystals.
The quantity of these impurities can be reduced by covering potassium chloride with liquids in which potassium chloride does not dissolve. The covering operation is usually effected in two stages. Water is added first to dissolve the impurity constituents and to form a cover liquor, which in an additional operation is separated from the underlying solids, i.e. the potassium chloride (Freiberger Forschungshefte, A 267, pages 191/192, Leipzig 1963). The covering effect of the cover liquor is, however, reduced by the presence of magnesium chloride and magnesium sulfate (Winnacker-Kuchler, Chemische Technologie, Vol 1, 3rd edition, page 127, 1970).